Optical device and methods

ABSTRACT

Methods and devices for manipulating optical signals. In one example, a LCOS (liquid crystal on silicon) device includes a surface bearing an anti-reflection structure. The anti-reflection structure includes i) a physical surface having a topography with features having lateral dimensions of less than 2000 nm and having an average refraction index which decreases with distance away from the surface; and ii) a configuration of the topography, averaged over lateral dimensions of greater than 2000 nm, varies with lateral position on the surface.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/374,628, filed Jul. 25, 2014, entitled OPTICAL DEVICE ANDMETHODS, which is a national stage entry of PCT/GB2013050142, filed Jan.23, 2013, which claims priority to GB 1201190.4, filed Jan. 25, 2012,the entire disclosures of each of which are herein incorporated byreference.

FIELD OF THE INVENTION

Embodiments of the present invention relate to methods and apparatus formodifying the phase of a beam of light for example for routing and/orswitching beams of light and adaptive optics. In particular, embodimentsof the present invention relate to polarisation insensitive phasemodification of beams of light.

BACKGROUND

Phase modulation of coherent light allows high-efficiency holographicimage projection and beam steering; the latter applications includeOptically Transparent Switches for optical networks, for Add-DropMultiplexers for Wavelength Division Multiplexing (WDM) Telecomms, andTV multicast. Incident illumination from the incoming fibres can berandomly or variably polarised or polarisation multiplexed, and theoverall rotation of polarisation of the original signal into the fibremay vary with time or day, temperature, mechanical stress on the fibre,etc. To deflect and route these signals without unacceptable losses orcontinuous adjustment polarisation insensitive methods are needed.

One method of routing signals uses a Spatial Light Modulator (SLM) whichinstead of modulating luminance modulates the phase of the reflectedlight in the relative range 0 to 2π, and presents blazed gratings orsuitable holograms to steer the incoming signals to different outputports, e.g. using a Liquid-Crystal-on-Silicon (LCOS) backplane todisplay the phase hologram. The Liquid Crystal (LC) material may be anematic material, generating an analogue blazed grating (in which caseonly a single linearly polarised component of the signal is modulated),or a suitable ferroelectric LC which can be polarisation insensitive buthas the disadvantage that only binary phase gratings or holograms can beformed, resulting in an extra 3 dB routing loss. Background prior artcan be found in U.S. Pat. No. 5,319,492 and JP2002/357802A.

In general it is desirable to suppress unwanted reflections in suchdevices; for some applications such as telecoms this is particularlyimportant to suppress unwanted crosstalk. Background prior art relatingto anti-reflection structures can be found in: U.S. Pat. No. 7,542,197;GB2,430,048A; US2012/0057235; and WO2012/123713.

Random polarisation can be accommodated by splitting the incoming signalinto two orthogonal polarisation streams, routing each separately usingsuitably oriented nematic SLMs, and recombining them (with additionallosses and a requirement for careful path length balancing), or by usinga technique such as an internal quarter-wave plate.

Light travelling through a medium can be disrupted by variations inrefractive index—e.g. in the atmosphere due to turbulence givingpressure changes or convection caused by temperature changes. For anyobject viewed in the far field, this alters the shape of the plane waveacross the entry optics, and limits the resolution of the optics belowthe theoretical limit. E.g. for an astronomical telescope this gives ablurred jittering image of a star which should be a point source. Forplanetary or surveillance images this also gives instantaneous spatialdistortion.

Adaptive optics can partially compensate for the disturbance, usually byusing a deformable mirror with an array of electro-mechanical actuators.These can correct the wavefront deformation of a few microns andpartially restore the wavefront.

SUMMARY OF INVENTION

According to a first aspect of the present invention, there is providedan optical routing device comprising a substrate; a plurality ofreflective pixel electrodes disposed on the substrate; a transparentlayer comprising at least one counter electrode; and a variablerefractive index layer disposed between the plurality of transparentpixel electrodes and the transparent layer. The variable refractiveindex layer comprises a material having a first, isotropic refractiveindex under no applied electric field and a second, different refractiveindex under an applied electric field wherein the second, differentrefractive index is isotropic perpendicular to the direction of theapplied electric field.

Embodiments of the present invention may be used to switch an opticalsignal between different outputs by applying spatial patterns ofvoltages between the pixel electrodes and the counter electrode. Thedifferent spatial patterns of voltages cause the variable refractiveindex layer to provide different diffraction gratings. The diffractiongratings may be configured to direct the input signal to differentoutput channels or fibres.

Embodiments of the present invention have the advantage that since therefractive index of the variable refractive index layer is isotropicperpendicular to the direction of the applied electric field and becausethe applied electric field is applied substantially in the directionthat the input signal is incident on the device, the routing device ispolarisation independent.

In embodiments of the present invention, the transparent layer comprisesa plurality of counter electrodes.

In embodiments of the present invention the pixel electrodes arearranged in rows and the plurality of counter electrodes each oppose arow of pixel electrodes. The pixel electrodes may be substantiallyrectangular and have a smaller dimension along the axis of the rows thanperpendicular to the axis of the rows.

According to an embodiment of the present invention, the variablerefractive index layer comprises a blue phase liquid crystal material.

According to a second aspect of the present invention, there is providedan optical telecommunications polarisation-insensitive beamswitching/routing device. The device comprises: a backplane comprisingdrive electronics configured to drive a plurality of opticallyreflective pixel electrodes for pixels of the device; a transparentlayer bearing at least one transparent counter-electrode; and a layer ofadjustable refractive index material between said counter-electrode andsaid pixel electrodes. The material has a first, isotropic refractiveindex, n, under no applied electric field and wherein under an appliedelectric field said material has a second, different refractive index,n′, wherein said second, different refractive index is isotropicperpendicular to a direction of said applied electric field.

According to an embodiment of the present invention the material is anon-liquid crystal material. An example of such a material isnitrobenzene.

According to an embodiment of the present invention the material is ablue-phase liquid crystal material and said backplane is an LCOS (liquidcrystal on silicon) backplane.

The liquid crystal material may have an optical Kerr constant of greaterthan 5 nm/V².

According to an embodiment of the present invention, the devicecomprises means to apply an ac bias voltage to one or both of saidcounter electrode and a set of said pixel electrodes. In an embodimentthe ac bias voltage is at least 20V peak-to-peak.

In an embodiment the LCOS backplane incorporates a temperature sensitiveelement, the device further comprising a temperature control systemcoupled to said temperature sensing element to control the temperatureof said material.

According to an embodiment of the present invention, the counterelectrode is segmented into rows.

According to a third aspect of the present invention, there is provideda method of routing an optical signal. The method comprises providing arouting device comprising a layer of blue phase liquid crystal between aplurality of pixel electrodes and at least one counter electrode;applying a first spatial pattern of voltages between the pixelelectrodes and the counter electrode such that the refractive index ofthe layer of blue phase liquid crystal varies spatially to provide afirst diffraction grating that deflects the optical signal to a firstoutput; and applying a second spatial pattern of voltages between thepixel electrodes and the counter electrode such that the refractiveindex of the layer of blue phase liquid crystal varies spatially toprovide a second diffraction grating that deflects the optical signal toa second output, thereby switching the optical signal from the firstoutput to the second output.

Embodiments of the invention allow polarisation independent routing withdevices as described above.

In an embodiment, the method comprises applying a square wavealternating voltage to the at least one counter electrode; and applyinga spatial pattern of drive voltages to the pixel electrodes, wherein thespatial pattern of drive voltages has a transition from a first set ofdrive voltages to a second set of drive voltages which coincides with atransition of the square wave alternating voltage and an instantaneouslevel of the square wave alternating voltage, at a first time, and saidfirst set of drive voltages provide the first spatial pattern ofvoltages and an instantaneous level of the square wave alternatingvoltage, at a second time, and said second set of drive voltages providethe second spatial pattern of voltages.

In an embodiment applying a first spatial pattern of voltages betweenthe pixel electrodes and the counter electrode comprises applying afirst drive signal to the pixel electrodes and applying a second spatialpattern of voltages between the pixel electrodes and the counterelectrode comprises applying a second drive signal to the pixelelectrodes.

In an embodiment, the method further comprises applying an offset signalto the counter electrode. In an embodiment, the first drive signal andthe second drive signals comprise a variable amplitude square wave.

In an embodiment, the method, further comprises applying a square wavealternating voltage to the counter electrode.

It has been observed that the refractive index certain of variablerefractive index materials depends on the square of the voltage. Byapplying a voltage to both the counter electrode and the pixelelectrodes and by timing the transitions of the voltages on the counterelectrode with the transitions from the first drive signal to the seconddrive signal, the voltage applied across the variable refractive indexmaterial can be increased and the change in refractive index for a givenchange in pixel voltage can be maximised.

According to a fourth aspect of the present invention, there is provideda method of polarisation-insensitive switching or routing of one or moreoptical telecommunications signal beams. The method comprises: providinga reflective liquid crystal on silicon backplane having a plurality ofpixels with a layer of blue-phase liquid crystal material over saidbackplane and a counter-electrode over said blue-phase liquid crystalmaterial; and displaying at least one diffractive optical element byapplying a patterned electric field to said blue-phase liquid crystalmaterial, wherein said electric field runs between said pixels of saidbackplane and said counter electrode and substantially parallel to lightincident onto and reflected from said reflective LCOS backplane; anddeflecting a said signal beam using said refractive optical element.

In an embodiment the diffractive optical element is configured todeflect a said signal beam in two dimensions.

In an embodiment said diffractive optical element comprises a hologram,the method comprising switching or routing a plurality of said signalbeams simultaneously using said hologram.

In an embodiment the signal beam comprises a wavelength divisionmultiplex (WDM) signal beam, the method further comprisingde-multiplexing said WDM signal beam into a plurality of separatewavelength signal beams, wherein said de-multiplexing retains componentsof two orthogonal polarising components in a said separate wavelengthsignal beam; directing said separate wavelength signal beams todifferent spatial regions of said diffractive optical element;deflecting said separate wavelength beams; and re-multiplexing at leastsome of said deflected separate wavelength beams.

In an embodiment said modulating using said electric field comprisesapplying a voltage of less than 25V between a pixel electrode of saidLCOS backplane and said counter-electrode.

According to a fifth aspect of the present invention, there is provideda method of manipulating an optical signal. The method comprisesproviding a device comprising a layer of blue phase liquid crystalmaterial in the path of the optical signal. By applying dynamicallyvarying spatial pattern of voltages to the layer of blue phase liquidcrystal material, the refractive index of the layer is caused to varyspatially according to the applied pattern.

In an embodiment, the optical signal is incident on the layer of bluephase liquid crystal at an angle of less than 5 degrees to normal. Theinventors of the present invention have demonstrated that at angles ofless than 5 degrees to normal, polarisation independent manipulation canbe achieved with a variation in polarisation of less than λ/8.

The voltages applied to the blue phase liquid crystal material may begreater than 50 volts and preferably greater than 100 volts.

In an embodiment the device comprises a plurality of electrodes disposedon one side of the layer of blue phase liquid crystal material thedynamically varying spatial pattern of voltages is applied to theplurality of electrodes.

In an embodiment the device comprises a photoconductive layer andapplying the dynamically varying spatial pattern of voltages comprisesapplying a light beam having a dynamically varying spatial pattern ofintensities to the photoconductive layer such that the resistance acrossthe photoconductive layer varies spatially.

Embodiments of the present invention employ optical addressing of theblue phase liquid crystal material layer.

In an embodiment, the method comprises measuring the optical signal todetermine a disturbance in a wavefront of the optical signal anddetermining the dynamically varying pattern to reduce the disturbance.Such an embodiment provides polarisation independent adaptive optics.

According to a sixth aspect of the present invention, there is providedan optical element. The optical element comprises a first transparentelectrode; a second transparent electrode; a variable refractive indexlayer disposed between the first and second transparent electrodes; anda photoconductive layer disposed between the second transparentelectrode and the variable refractive index layer. The variablerefractive index layer comprises a material having a first, isotropicrefractive index under no applied electric field and a second, differentrefractive index under an applied electric field wherein the second,different refractive index is isotropic perpendicular to the directionof the applied electric field.

In an embodiment the variable refractive index layer comprises a bluephase liquid crystal material.

In an embodiment the element further comprises a light blocking layerbetween the variable refractive index layer and the photoconductivelayer.

According to a seventh aspect of the present invention there is providedan adaptive optics system comprising an optical element as describedabove.

Features of the above-described aspects and embodiments of the inventionmay be combined in any permutation.

Anti-Reflection Structures/Coatings

In some preferred embodiments of each of the above described aspects ofthe invention the (routing) device or optical element incorporates oneor more layers with an anti-reflection (AR) coating, for example on anupper transparent (optical input/output) layer of the device/element.Additionally or alternatively one or more internal interfaces of thedevice/element may also incorporate an anti-reflection coating. In someparticular implementations, the characteristics of the AR coating varyover a surface of the device, in particular to optimise differentspatial regions of the device/element for different operatingwavelengths of the device/element. This is particularly advantageous inan optical system incorporating a wavelength-selective opticaldemultiplexer, to direct different optical wavelengths to the differentspatial regions of the device/element optimised for those wavelengths.

In addition a diffraction pattern displayed on the device mayincorporate a component to generate a reflection-cancelling beam, topartially or substantially wholly cancel a reflection from either aninternal or external surface interface of the device/element. Preferablythe diffraction pattern is configured so that the reflection-cancellingbeam is generated by a second (and/or higher) diffraction order (thefirst diffraction order being used for the intended optical wavefrontmanipulation, for example routing).

The skilled person will appreciate that these latter techniques may beemployed independently of the above described aspects of the invention.

Thus in a further aspect the invention provides an LCOS (liquid crystalon silicon) device comprising a surface bearing an anti-reflectionstructure, wherein: i) the anti-reflection structure comprises aphysical surface having a topography with features having lateraldimensions of less than 2000 nm and having an average refraction indexwhich decreases with distance away from said surface; and ii) aconfiguration of said topography, averaged over lateral dimensions ofgreater than 2000 nm, varies with lateral position on said surface.

In embodiments the surface topography comprises a pattern of features,in particular a regular/cyclical pattern of varying height or reliefover the surface. Laterally the pattern repeats or has a characteristiclength scale (in the case of a random pattern) which is less than thewavelength at which the device operates, for example less than 2000 nm,1900 nm, 1800 nm, 1700 nm or 1600 nm. However the anti-reflection layeralso has one or more parameters of the configuration or structure (forexample, average feature pitch and/or feature shape) which, whenaveraged over distances greater than a wavelength, varies with positionon the surface: that is there is a macroscopic variation of thetopography. This allows different surface regions of the device to beoptimised for different wavelengths, which is useful in many LCOSapplications including, but not limited to: opticalswitching/routing/pulse shaping, holography for example for displayingimages, the display of diffraction patterns/holograms for opticaltweezers, and more generally any application where wavefront control isdesired.

In preferred implementations the surface bearing the topography is afront surface of the device, more particularly a coverplate of thedevice, although the skilled person will recognise that this is notessential.

In a related aspect the invention provides an LCOS device, in particularas claimed in claim 34 or 35, combined with a controller to display ahologram on said LCOS device to deflect first light into a firstdiffractive order of said hologram, wherein said hologram is furtherconfigured to deflect second light into a second diffraction order ofsaid hologram, wherein said LCOS device has an interface generatingunwanted reflected light, and wherein said second light is in antiphasewith said unwanted reflected light.

The features of such an LCOS device may be combined with or employedseparately to the laterally varying surface topography pattern mentionedpreviously. Broadly speaking in embodiments, because the firstdiffractive order is employed for directing the light in whatever way isdesired (for example, according to the application as describedpreviously), a second and/or higher diffraction order may be employed tocancel one or more unwanted beams reflected from an internal interface.Thus in embodiments at least a proportion of the second light, andpreferably substantially all the second light, is in antiphase with thereflected beam to be cancelled. Preferably the second(reflection-cancellation) beam has substantially the same power as theunwanted reflected light in that direction (the same amplitude and/orintensity). The skilled person will recognise, however, that somebenefit may be obtained from the technique even where the beams are notexactly at the same power or exactly in antiphase.

The skilled person will appreciate that because holograms are additivein nature it is straightforward to implement the reflection cancellationbeam: to a desired hologram displayed on the SLM it is simply necessaryto add a second hologram (adding the respective pixel values) whichgenerates a second (or higher) order beam with the desired power(amplitude/intensity), phase, and direction. The skilled person will beaware of many algorithms which may be employed to perform such acalculation. The power, phase and direction of the unwanted reflectionto be cancelled can be readily obtained by numerical modelling of theLCOS device using any of a number standard optical modelling packages—ingeneral the precise amplitude, phase and direction of a reflection to becancelled will depend upon the detail structure of the LCOS device.

As previously mentioned either or both of the above approaches may beemployed to optimise the anti-reflection properties at differentrespective wavelengths in different respective lateral surface regionsof the device.

In a related aspect the invention provides a method of suppressing anunwanted reflection in a spatial light modulator (SLM) comprising areflective liquid crystal on a silicon backplane having a plurality ofpixels with a layer of blue-phase liquid crystal material over saidbackplane and a counter-electrode over said blue-phase liquid crystalmaterial, the method comprising using said SLM to deflect light into asecond or higher diffraction order of a diffraction pattern displayed onsaid grating, wherein said second or higher order deflected lightdefines a reflection-cancellation beam in antiphase with said unwantedreflection.

Preferably, as previously described, the SLM is an LCOS SLM. Preferablythe unwanted reflection comprises an internal reflection at an interfaceof the device, although in principle the technique is not restricted tosuch an internal reflection. The interface may be an internal interface,for example a coverplate/liquid crystal interface such as an interfacecomprising one or more of a liquid crystal layer, an alignment layer, anelectrode layer, and a coverplate layer. The skilled person willappreciate that, depending upon the configuration of the device and/orthe thickness of the various layers, the interface may or may notinclude all of these. For example, in a blue phase device the alignmentlayer is not required. The reflection cancellation need not be complete,although preferably it is substantially complete.

In a still further related aspect the invention provides a spatial lightmodulator (SLM) in combination with an SLM controller, said SLM havingan interface generating an unwanted reflection, wherein said SLMcontroller is configured to drive said SLM to deflect light into asecond or higher diffraction order of a diffraction pattern displayed onsaid grating, wherein said second or higher order deflected lightdefines a reflection-cancellation beam in antiphase with said unwantedreflection.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments of the invention will be described, by wayof example only, with reference to the accompanying drawings in which:

FIG. 1 shows a schematic cross section of an optical routing device;

FIG. 2 shows a graph illustrating the behaviour of a blue phasestructure under applied electric field;

FIG. 3 shows the phase shift of light transmitted through a cell as afunction of voltage for three wavelengths and both polarisations;

FIG. 4 shows the cell and detected intensity of light detected;

FIG. 5 shows voltage schemes for driving liquid crystal material on topof an LCOS backplane;

FIG. 6 shows a phase/voltage curve for a cell;

FIG. 6A shows an example phase/voltage curve for a cell;

FIG. 7 a driving scheme for a cell having a controllable variableamplitude square wave drive with a separately defined offset;

FIGS. 8 and 9 show a system diagram for a multiple wavelengthpolarisation independent router;

FIG. 10 shows a switching device with a segmented front electrode;

FIG. 11 shows a switching device mounted on a substrate;

FIG. 12 shows a cross section of an optical device;

FIG. 13 shows an optical device in use;

FIG. 14 shows an apparatus for modifying the phase of a beam of light;

FIG. 15 shows an adaptive optics system applied to a telescope;

FIG. 16 shows a system for modifying the phase of a beam of light in atransmissive configuration;

FIG. 17 shows a graph of phase shift as a function of angle of incidencefor normal and oblique incidence;

FIG. 18 shows the maximum variation of the phase shift as a function ofthe incidence angle;

FIG. 19 shows a schematic diagram of a LCOS SLM;

FIG. 20 shows an illustration of a problem with reflection from a frontcoverplate surface;

FIG. 21 shows an example of light being dispersed across the face of aLCOS SLM;

FIGS. 22A to 22C illustrate three potential coating schemes, showing (A)thin film dielectric layers, (B) a graded index layer, and (C) patternedmicrostructures;

FIG. 23 shows an illustration of an effective medium representation of astructured anti-reflective surface; and

FIG. 24 shows a block diagram of an SLM controller/driver configured todrive an SLM with a reflection-compensation hologram.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a schematic cross section of an optical routing device 100.The device is formed on a substrate 102. A plurality of opticallyreflective pixel electrodes 104 are arranged on the substrate 102. Alayer of adjustable refractive index material 106 is arranged over thepixel electrodes 104. A transparent counter electrode 108 is arrangedover the layer of adjustable refractive index material 106.

The adjustable refractive index material 106 has a refractive indexwhich varies with the electric field applied across it. An example ofsuch a material is blue phase liquid crystal. The adjustable refractiveindex material 106 has an isotropic refractive index in the directionperpendicular to the applied electric field.

In the device shown in FIG. 1, by applying a spatially varying patternof voltages between the electrodes 104 and the counter electrode 108,the refractive index of the adjustable refractive index material 106 canbe caused to vary spatially. In order to direct a beam of light incidenton the upper surface of the counter electrode 108, a first pattern ofvoltages is applied across the adjustable refractive index material 106that cause its refractive index to vary spatially to form a firstdiffraction grating. The first diffraction grating deflects the incidentbeam of light in a first direction. To switch the beam of light to asecond direction, the applied pattern of voltages is changed to a secondpattern of voltages that cause the refractive index of the adjustablerefractive index material 106 to vary spatially to form a seconddiffraction grating.

By switching between different patterns of voltages as described above,an incident beam of light can be switched between different outputs.

If a material that has an isotropic refractive index in the directionperpendicular to the applied electric field is used, the switching isindependent of the polarisation of the incident beam.

In the following, the wording isotropic perpendicular to the appliedelectric field is used to describe that there is substantially nodifference in refractive index in any direction perpendicular to theapplied electric field, the refractive index may vary in any otherdirection.

FIG. 2 shows the behaviour of a blue phase structure under appliedelectric field. The blue phase liquid crystal has symmetrical domains ofliquid crystal molecules arranged in a structure which, with no appliedelectrical field, is optically isotropic. A blue phase liquid crystalmaterial is a liquid crystal material in a blue phase. Examples of bluephase liquid crystal materials stable over a wide temperature range canbe found in US2009/0115957, to which reference may be made—thisdescribes materials stable over a range of greater than 35° C. and/orwhich are stable at room temperature.

Deforming the domains (e.g. by applying an electric field) along thepropagation direction of the light can be used to modulate polarisedlight, since the deformed domain undergoes a change in opticalrefractive index and becomes optically anisotropic. If the deformationis along the light transmission axis so that the deformed domain isstill symmetrical normal to the light transmission, a localised changein refractive index occurs which is independent of the incidentpolarisation angle, and can be used to produce SLMs capable ofpolarisation independent phase modulation.

The behaviour discussed above was experimentally demonstrated asfollows. A liquid crystal LCM-PSBP.1328UV (from LC Matter Corp.) wasused for the experiment. A 12 micron thick glass cell with ITOelectrodes and no other alignment layers was filled with the mixture.Square waveform voltage with frequency of 1 kHz was applied across thecell in order to introduce anisotropy in the Blue Phase.

FIG. 3 shows the phase shift of light transmitted through the cell as afunction of voltage for three wavelengths and both polarisations. Phaseshift was identical for both polarisations within the range of appliedvoltage for any wavelength. For wavelength of 480 nm, a 68 volt squarewave at 1 kHz gave a phase shift as shown of greater than 2π.

Phase shift was measured using a cell as a Fabry-Perot interferometer.The cell and detected intensity are shown in FIG. 4. The spectrum of thelight transmitted through the cell showed a characteristic interferencepattern due to the direct and double-reflected light in the cell. Thepositions of the interference peaks depend on the refractive index ofthe material and move as voltage applied across the cell.

The high voltage required to get 2π phase shift reflects the fact thematerial has low Kerr constant (K) of 1.6 nm/V2 at 600 nm. The drivevoltage and cell thickness should be reduced considerably by usingmaterials with the large Kerr constant (K up to 12.7 nm/V2) that havebeen recently developed for in-plane switching display applications. Forthis material a 2π phase shift at 600 nm wavelength should be obtainedwith a 10 μm reflective cell and driving voltage of ±14V, well withinthe range of LCOS backplanes.

Thus, the inventors have demonstrated a method for phase modulation ofrandomly polarised light.

It is envisaged that the liquid crystal material may be chosen in orderto get a wide temperature range blue phase. Mixtures of a liquid crystaland a polymer have been shown to stabilise the blue phase in atemperature range from 60 C to −10 C degrees. In order to stabilise theblue phase in a temperature range, the blue phase material may be dopedwith nanoparticles. This improves the sensitivity of the blue phasematerial to electric fields.

Another possibility is the use of bimesogenic liquid crystals.

The Blue Phase has the additional advantage that no alignment or rubbinglayers are required. This makes possible simplifiedpolarisation-independent telecoms routing.

FIG. 5 shows two ways of driving the liquid crystal (LC) material on topof the LCOS backplane. The first driving scheme is an analogue variableamplitude square wave drive. Here the counter electrode which is thefront electrode (FE) of the LCOS device is maintained permanently athalf the supply voltage of the pixel driver transistors, and a variableamplitude square wave is applied to the pixels to give a symmetricaldc-balanced square wave of adjustable amplitude across the pixels. Themean RMS voltage determines the state of the LC.

The second method, digital variable mark/space ratio drive, involvesapplying a square wave to the front electrode of the same amplitude asthe supply voltage, and the pixels are switched at different timesbetween the supply or ground. This allows a dc-balanced butasymmetrically timed waveform across the LC which has a variable RMSvalue, to which the LC responds, giving an average response similar tothe fixed FE version, but with small variations as the LC follows thewaveform.

The latter method is satisfactory for image projection applications, butcauses unacceptable phase flicker which decreases the efficiency ofphase holograms. However it allows twice the drive voltage across theliquid crystal from the same backplane process, or for the same voltagerequires a lower voltage process, allowing smaller transistors andtherefore higher integration, lower power, and lower cost.

Since rows of pixels are usually loaded sequentially down the pixelarray a combination of the two causes progressive loss of drive andincreased phase flicker down the pixel array.

The blue phase liquid crystal responds more rapidly to the appliedelectric field than a nematic phase liquid crystal. As shown in FIG. 3,the refractive index change is proportional to the square of the appliedfield. This means that for a given backplane voltage swing the phasemodulation may be quite small, and may require a very high drivevoltage.

FIG. 6 shows a phase/voltage curve for a cell. If the backplane driveswing can be pushed further up the response curve, it is possible to getan improved phase modulation for the same backplane swing. As can beseen in FIG. 6, by offsetting the B/P Drive voltage, the phase responsecan be increased from the range shown as ‘Phase no offset’ to the range‘Phase with offset’. This can be implemented by “overswitching” thefront electrode above the supply rail and below ground to produce anoffset added to the backplane drive voltage, but only by using thesecond drive scheme described above. A combination of the first andsecond schemes allows this; rather than switch between the supply rails,the pixels are switched at the FE transition time to different analoguevoltages, thus giving a controllable variable amplitude square wavedrive with a separately defined offset.

FIG. 6a shows a graph of phase modulation in steps of pi against drivevoltage across a cell. The phase modulation is proportional to thesquare of the voltage applied across the cell. FIG. 6a shows examplevalues for the voltage and phase modulation to demonstrate the effect ofoffsetting the voltage on the phase change.

As can be seen in FIG. 6a , from 0 volts a drive voltage of 82 volts 602is required to give a phase shift of 2pi 604. However, if an offset of82 volts is applied, if takes only 35 volts 608 of change to give thesame differential phase modulation 606. If the front electrode isswitched above and below the supply rail by this voltage, the modulationrequired to drive the pixel is reduced from around 80 volts to around 35volts.

FIG. 7 shows such a driving scheme. The front electrode (FE) voltage is“overswitched” above and below the VLSI supply voltage and Ground. Thisallows the full VLSI voltage range to be used for phase control furtherup the phase/voltage curve shown in FIG. 6.

However for this to work on a large 2-dimensional pixel array, all therows of pixels have to be loaded on the FE transition, otherwise phaseflicker will be reintroduced which will get progressively worse down therows of refreshed pixels.

The chip can be designed so that rows of rectangular pixels forming1-dimensional arrays, suitable for grating-type holograms, can bedriven, with a horizontally segmented FE. This allows a separate driverfor each FE segment, ensuring that the FE switches at the same time thatthe pixels are refreshed, eliminating the phase flicker, and allowingthe LC drive to be offset up the response curve.

For a multiple wavelength routing device a system can be designed toensure that the separated wavelengths fall onto the required segments.Such a system requires a reasonably polarisation independent wavelengthsplitting grating and allows a single-device polarisation independentrouter.

FIGS. 8 and 9 show a system diagram for a multiple wavelengthpolarisation independent router 800. FIG. 8 is a side-view and FIG. 9 isa top view.

The router 800 switches signals between fibres of a fibre array 802. Aninput signal is received in an input fibre 802. This is switched to oneor more output fibres depending on the state of the switch. The inputsignal is passes through a lens 806 and is wavelength demultiplexed bywavelength demultiplexer 808. Different wavelengths are directed todifferent areas of the LCOS device 812. The LCOS device 812 applies ahologram/grating which deflects the wavelength incident on thatparticular area in the direction orthogonal to the wavelength splittingaxis. The beam is reflected through a wavelength multiplexer 814 whichcollects together all the wavelengths which have been deflected by agiven angle and these are then focussed by a lens 816 into thecorresponding output fibre 818. The angle of deflection (inverselyproportional to grating pitch) determines the destination fibre.

This LCOS device can be segmented by having strips of ITO on the coverglass each corresponding to a separate wavelength channel, so that eachwavelength can be driven independently of other wavelengths.

FIG. 10 shows a switching device 1000 with a segmented front electrode.The LCOS device 1010 has a front electrode divided into strips 1012.Each strip 1012 covers a plurality of electrodes 1014 on the opposingside of the variable refractive index material. The wavelengthdemultiplexer directs different wavelengths of the input signal 1018 todifferent strips. The pixel dimensions on the LCOS backplane are between1.6 and 15 microns.

Beneath each front electrode strip there is a one-dimensional striphaving the same width as the front electrode strip. The pixels may berectangular as shown in FIG. 10, or may be square.

The segments which form the diffraction grating can be up to 20 mm highand unto 100 microns wide.

FIG. 11 shows a switching device 1100. The device is mounted on asubstrate 1102. A temperature control may be attached to the substrate1102 which controls the temperature of the device so that the liquidcrystal material is maintained in the blue phase. A flexible PCB 1104 ismounted on the substrate 1102. An active backplane 1106 is also mountedon the substrate 1102. The active backplane 1106 runs under a layer ofblue phase liquid crystal material. Bonding wires 1108 connect theactive backplane 1106 to the flexible PCT 1104. A front glass cover 1110is located over the layer of blue phase liquid crystal material. Theglass cover carries stripe front electrodes 1112 which run across theblue phase liquid crystal material. The flexible PCB runs beside theedge of the blue phase liquid crystal material and the glass overoverlaps the flexible PCB 1112, this allows connections between theflexible PCB 1104 and the front electrodes 1112 to be made.

The front electrode connections are made by silver loaded epoxy from theITO front electrodes on the front glass cover.

FIG. 12 shows a cross section of an optical device 1200. The device hasa transparent substrate 1202. The transparent substrate is formed fromglass. A transparent electrode formed by transparent conducting oxidelayer 1204 is coated on the substrate 1202. The transparent conductingoxide layer is, for example, formed from Indium tin oxide. Beneath thetransparent conducting oxide layer 1204 there is a layer of adjustablerefractive index material 1206. The layer of adjustable refractive indexmaterial 1206 is, for example a layer of blue phase liquid crystal asdescribed above. Beneath the adjustable refractive index layer 1206,there is a reflective layer 1210. the reflective layer is formed fromfor example, 15 layers of zinc sulfide and magnesium fluoride. Beneaththe light blocking layer 1210 there is a layer of a photoconductivematerial 1208. The photoconductive layer 1208 is, for example, formedfrom hydrogenated amorphous silicon or cadmium sulfide. Beneath thephotoconductive layer 1208, there is a second transparent electrodewhich is formed by transparent conducting oxide layer 1212 coated on asecond transparent substrate 1214. The first and second electrodes areplanar and occupy planes that are parallel and spaced apart with thephotoconductive layer and the adjustable refractive index material aredisposed between the first and second electrodes.

FIG. 13 shows the optical device 1200 in use. A voltage waveform 1220 isapplied between the electrodes formed by the transparent conductingoxide layers 1204 and 1212. A write beam 1230 of light which varieslaterally across the device is applied to the device through thesubstrate 1214. The write beam 1230 causes the conductivity of thephotoconductive layer 1208 to vary spatially. Since the conductivity ofthe photoconductive layer 1208 varies spatially, the voltage across theadjustable refractive index layer 1206 will vary spatially correspondingto the spatial variation in the write beam 1230.

A read beam 1240 is incident on the substrate 1202 on the side of thedevice including the adjustable refractive index layer 1206. Thevariation in the voltage across the variable refractive index layer 1206causes the refractive index of the variable refractive index layer tovary spatially. Therefore, the phase of the outgoing light 1242reflected from the reflective layer 1210 can be modified. The reflectivelayer 1210 stops the write beam from passing through the device and theread beam from activating the photoconducting layer.

The voltage waveform 1220 may take the form described in reference toFIGS. 5 to 7 above. It is noted that an optically driven device as shownin FIGS. 12 and 13 may be driven with higher voltages than thoseavailable from an LCOS backplane.

In place of or in addition to the reflective layer, the device may alsocomprise a light blocking layer. Such a light blocking layer could bearranged below the reflective layer and configured to absorb the 1-5% ofresidual light and stop it hitting the photoconductive layer.

In an alternative embodiment, the device may be used in a transmissiveconfiguration. In this configuration, there is no reflective layer andthe read beam of light passes through the device. In such an embodiment,different wavelengths of light are used for the read and write beams oflight or each beam is pulsed in a different part of the voltagewaveform.

FIG. 14 shows an apparatus for modifying the phase of a beam of lightusing the device 1200 described above with reference to FIGS. 12 and 13.The apparatus has a light source 1260 such as a laser or high brightnesslight emitting diode. The light from the light source is collimated by alens 1262 onto a beamsplitter 1264. The beamsplitter directs the lightonto a microdisplay 1266. The microdisplay 1266 may be a liquid crystalon silicon display or a digital light processing display. Themicrodisplay 1266 sets the pattern of the write beam which will beincident on the device 1200. The signal from the microdisplay 1266passes back through the beamsplitter 1264 and then through a relay lens1268. The signal is imaged on the transparent substrate 1214 of thedevice 1200. The spatial variation of intensity of the write signalcauses the resistance of the photoconductive layer 1208 to varyaccording to the electrical signal that is input to the microdisplay1266. This variation causes the voltage across the variable refractiveindex material 1206 caused by the voltage between the electrodes 1212and 1204 to vary. A read light, for example a signal to be routed or asignal having its wavefront modified is incident upon the substrate 1202on the opposite side from the side on which the write signal isincident. The read signal is modified by the refractive index of thevariable refractive index layer 1206, however since the refractive indexof the variable refractive index material is isotropic in the directionperpendicular to the applied electric field, the modification of theread signal is polarisation independent.

The device described above has applications in adaptive optics. Lighttravelling through a medium can be disrupted by variations in refractiveindex—e.g. in the atmosphere due to turbulence arising from pressurechanges or convection caused by temperature changes. For any objectviewed in the far field, this alters the shape of the plane wave acrossthe entry optics, and limits the resolution of the optics below thetheoretical limit. E.g. for an astronomical telescope this gives ablurred jittering image of a star which should be a point source. Forplanetary or surveillance images this also gives instantaneous spatialdistortion.

Adaptive optics can partially compensate for the disturbance, usually byusing a deformable mirror with an array of electro-mechanical actuators.These can correct the wavefront deformation of a few microns andpartially restore the wavefront.

FIG. 15 shows an adaptive optics system applied to a telescope. Thetelescope 1500 has a concave mirror 1502 and a convex mirror 1504. Thewaveform 1506 of light coming into the telescope 1500 has fluctuationsdue to atmospheric disturbance. The fluctuating waveform is focussed bythe mirrors 1502 1504 to give an input waveform 1508.

The input waveform 1508 is partially split by a beamsplitter 1510. Partof the beam is incident upon a wavefront sensor 1512 and the remainingpart of the beam is incident upon an active phase correction device 1514which comprises a layer of variable refractive index material such asblue phase liquid crystal as described above. The output of thewavefront sensor 1512 is used by an adaptive optics correctioncontroller 1516 to control the spatial variation of the voltage appliedacross the variable refractive index material and thereby control thephase correction. The corrected beam 1518 is output to imaging optics ofthe telescope.

Adaptive optics can also be used to correct for aberrations in opticalsystems, improving vision to a greater extent than lens correction, ande.g. for laser surgery, when the imperfections of the eye's lens must becorrected for retinal welding, etc.

Many adaptive optics applications require polarisation independence anda rapid (millisecond) response time, the devices described above aresuited to these applications since they allow for polarisationindependent phase correction without a requirement for moving parts suchas adjustable mirrors.

FIG. 15 shows an open-loop system, however, those of skill in the artwill recognise that the devices described above could also be applied toa closed loop system.

FIG. 16 shows a system for modifying the phase of a beam of light whichhas an active phase correction device in a transmissive configuration. Abeam of light from a source 1702 is broadened and deflected byatmospheric interference to give waveforms 1704 with fluctuations 1704.The phase correction device 1706 corrects the phase of the beam to givea phase flat beam.

The devices described above provide polarisation independent phasemodulation for light beams of normal incidence. For beams with anon-zero angle of incidence, some polarisation dependence is introducedinto the phase change.

FIG. 17 shows phase shift as a function of the polarisation angle fornormal and oblique incidence. There is a phase change of up to Δ_(max)for light beams incident at a non-zero angle of incidence.

FIG. 18 shows Maximum variation of the phase shift as a function of theincidence angle. For normal incidence the polarisation ellipsoid is seenalong its long axis as a circle, therefore, the phase shift isindependent of the polarisation angle. For any oblique incidence thepolarisation ellipsoid is seen at some angle as an ellipse, therefore,the phase shift is now depends on the polarisation angle.

As can be seen from FIGS. 17 and 18, the angle of incidence ispreferably kept low to ensure that the phase shift is relativelypolarisation independent. If a variation of λ/8 is the acceptedvariation then the angle of incidence of the incoming beam should bekept within 5 degrees.

Anti-Reflection Structures/Coatings

We will now describe how the performance of blue phase and other LCOSSLMs can be enhanced using structured surfaces on the front SLMcover-plate to reduce reflections. This is particularly applicable towavelength selective switches, where a very low reflectivity over alarge bandwidth is desirable.

The example LCOS SLM of FIG. 19 has three components: a siliconbackplane, a coverplate with a common electrode on one surface, and aliquid crystal layer. The coverplate is typically float glass with athin-film anti-reflection coating on the front surface, and an ITOlayer. Optionally, a liquid crystal alignment layer may be provided onthe back surface, although this is not required for a blue phase device.Ideally, both the front coverplate surface and the coverplate/liquidcrystal interface (comprising glass, ITO, optional alignment layer, andliquid crystal layers) have zero reflectivity. Thus the wavefrontexiting the SLM is solely affected by the phase delay imparted acrossthe liquid crystal layer.

For most applications, the front surface reflectivity can besufficiently reduced through the use of standard thin film coatings.However, for telecom applications, such as the implementation ofwavelength selective switches based on beam-steering, the reflectivityof the front surface should preferably be reduced to R_(f)<0.01% over awide wavelength range in order to minimize crosstalk. To illustrate thisconsider FIG. 20, where a plane wave at an angle of θ_(i) is incident ona LCOS SLM that displays an ideal blazed grating of period T. A fractionof the signal beam (m=1) is reflected back towards the blazed gratingwhich is re-diffracted such that it travels in the same direction as them=2 order. The m^(th) diffraction order of the grating is diffractedthrough an angle of θ_(m) according to

$\begin{matrix}{{\sin\;\theta_{m}} = {{\sin\;\theta_{i}} + \frac{m\;\lambda}{T}}} & (1)\end{matrix}$where λ is the wavelength of the light and the angles are the valuesmeasured in air. For an ideal blazed grating, all the light isdiffracted into the m=+1 order. However, due to spatial and phasequantization of the grating, some light ends up in higher and symmetric(m≠1) orders. For certain switch geometries, light in these m≠1 orderscan couple into output ports leading to crosstalk. For practical telecomapplications the crosstalk should preferably be suppressed to <−40 dB.

Consider the case where θ_(i)=0. As mentioned, ideally all the light isdiffracted into the +1 order through an angle of θ₁. However, if thefront surface of the SLM has a finite reflectivity of R_(f), a portionof the +1 order will be reflected back towards the SLM at an angle −θ₁with respect to the normal. This order will be re-diffracted by thegrating such that it propagates at an angle of 2θ₁ to the normal, which,for small diffraction angles, corresponds to the angle of propagation ofthe m=+2 order of the original diffracted beam, θ₂. Assuming acoverplate with refractive index of n=1.5, and no anti-refectioncoating, the Fresnel reflection coefficient of the front coverplatesurface is approximately 4%. Let us assume an SLM which has 100%diffraction efficiency (no absorption losses and a perfect blazedgrating). The power in the m=+1 and m=+2 directions will therefore begiven by 0.96P_(in) and 0.04P_(in) respectively (ignoring multiplereflections). As a result, the theoretical crosstalk is −13.8 dB. Thiscompares well to values of −14 to −18 dB measured experimentally. If wereduce the front face reflectivity to 1% (typical of single layer thinfilm coatings), the crosstalk reduces to approximately −20 dB. To reducethe crosstalk to <−40 dB the front face reflectivity should preferablybe reduced to <0.01%. Such a low reflectivity is challenging,particularly as we should preferably ensure this value is maintainedacross the C-band, L-band, or C and L band. For example, the C-bandextends from 1530 nm to 1570 nm, and L-band from 1565 nm to 1625 nm, andan optimized multi-layer coating can expensive to fabricate.

FIG. 21 shows the functional operation of a LCOS SLM used in conjunctionwith a wavelength de-multiplexer. Such a configuration may be used inwavelength selective switches, where separate sub-holograms deflectspecific optical wavelengths, and in pulse shaping systems, where lightis dispersed across the face of the SLM and each wavelength band ismodified to produce the desired output pulse. Ideally we desire ananti-reflection coating that has continuously varying optical propertiesto match the incident wavelength.

Although particularly important for telecom applications, otherapplications that employ the optimization of complex hologram patternsalso benefit from a reduction of front face reflectivity. The quality ofthe replay field in display holography and optical tweezers will improveas reducing R_(f) ensures that the output wavefront more closely matchesthe designed wavefront.

It is desirable to provide an anti-reflection layer with the followingproperties:

-   -   a. Sufficient angular independence    -   b. Good polarization independence    -   c. Substantially no wavelength dependence over the desired        operational bandwidth    -   d. Low temperature sensitivity    -   e. Is cheap and robust    -   f. Can be optimized across the surface of a coverplate to match        operational wavelength range

There are four potential techniques which may be employed for reducingcrosstalk in practical LCOS SLM applications: geometric, thin filmdielectric coatings, graded index coatings, and patternednano-structures:

-   -   1) Geometrical—Rather than using a coverplate with parallel        surfaces, we use a wedged coverplate that reflects light away        from the main replay field. The applicability of this approach        depends on the actual optical geometry, and may not be practical        in some WSS (wavelength selective switch) architectures.    -   2) Thin film dielectric coatings—This technique for implementing        anti-reflection coatings is illustrated in FIG. 4a . Typical        C-band coatings available off-the-shelf can have a reflectivity        <0.1%. To reduce R_(f) to <0.01% on a complex substrate (ITO        layer and optional alignment layers on the reverse side) could        be done with an expensive custom coating run. However, as thin        film coatings exhibit polarization dependence if used off-axis,        it will be important to factor in the beam steering angles and        alignment tolerances when designing the coating. Finally, the        flexibility of thin film coatings is limited by the number of        suitable deposition materials available (refractive indices),        temperature stability, and suffer potential delamination        problems if environmental conditions change.    -   3) Graded index coatings—This refers to a class of coatings that        use an impendence matching layer with a graded change in        refractive index as illustrated in FIG. 4b . By continuously        increasing the refractive index from n_(air) to n_(s), where        n_(s) is the substrate refractive index, one can avoid sharp        dielectric boundaries that cause Fresnel reflection. The profile        can be, for example, parabolic or cubic, and can be implemented        by varying the packing density of the films or by using inclined        nano-rods.    -   4) Patterned nano-structures—This refers to a technique based on        patterned nano-structured surfaces, an example of which is        illustrated in FIG. 4c . The dimensions of nano-structures are        <λ, so the incident light sees an average refractive index at a        specific value of z that depends on the ratio of air and        substrate. Thus there is an effective gradually change in        refractive index through the patterned layer from n_(air) to        n_(s). The structures can be designed and optimized using an        effective medium theory (EMT) where the refractive index depends        on the topology of the material. There are two main approaches        for making such layers. The first is by imparting porosity to        the film, with the porosity decreasing as we approach the        substrate, for example by sol-gel processing. The second        approach involves fabricating arrays of nano-structures that        either suppress reflections by light trapping and        multiple-internal reflections (surface texture) or by using an        anti-reflection grating where the grating topology is engineered        to ensure that only the m=0 order of the grating propagates, and        that it produces a continuously varying refractive index. FIG.        23 illustrates this approach, with a patterned nano-structure        with N=6, showing the structured surface on the left and a        corresponding effective equivalent medium model on the right.        The effective index of the six layers are set such that        n_(air)=n_(a)<n₁<n₂<n₃<n₄<n₅<n₆<n_(s)=n_(substrate).

Graded Index/Patterned Nano-Structures for Telecoms Wavelengths

Raguin and Morris showed theoretically that a multi-level approximationto a pyramidal anti-reflection structure fabricated on a GaAs substrate(N=2, 4, and 8 phase-levels) and optimized for operation at 10.6 μmcould have a reflectivity <3×10⁻³% for all values of N for a randomlypolarized normally incident beam (D. H. Raguin and G. M. Morris,“Antireflection structured surfaces for the intrared spectral region”,Appl. Opt. 32, 1154-1166 (1993)). This exceeds the above target of0.01%. Moreover, their model predicted a T=99.9% transmission when N=8even if the wavelength range varied by 10.6 μm±10%, and the incidenthalf angle varied by 30°.

Experimental results for graded index coating and patternednano-structures have been presented for a variety of surface topologies,material systems, wavelengths and operating conditions (see thereferences in P. Lalanne and M. Hutley, “The optical properties ofartificial media structured at a subwavelength scale”). Surfacefabrication and replication techniques allow such surfaces to befabricated for the visible. For example, Hutley and Gombert have used UVembossing into plastic using a nickel master to produce AR coatedFresnel lenses for overhead projectors (M. Hutley and A. Gombert,“Moth-eyes: the tortuous path from a glint in the eye to a commercialreality”, Photonics Science News 6, 35-39 (2000)). Anti-reflectionnano-structures may be produced for the optical C- and L-bands bysimilar techniques.

Referring again to FIG. 21, one can further vary the graded ornano-structured surface topology across the coverplate surface (varyingpitch and/or shape, and the like), to thereby tailor the coating tomatch specific the wavelength at that section of the device. This is notpossible using standard thin film deposition processes. In addition, asurface relief profile is more rugged than complex thin film dielectricstacks that can de-laminate—a potential problem during operation atelevated temperatures (telecom systems typically run at 60° C. tosimplify temperature stabilization), and during LCOS SLM devicefabrication.

Example Nanostructured Surfaces

Broadly speaking, the aim of using a nano-structured anti-reflectioncoating is to make a more effective broadband coating for telecoms, andone that can be tailored so that the reflectivity is <0.01% at eachwavelength location across the SLM in a system that uses a wavelengthde-multiplexer. This will allow simplification of the hologram designand optimization by reducing the front surface reflections (so they maybe neglected), particularly coupled with the fact that blue phase SLMsdo not require an alignment layer, thereby simplifying calculation andmeasurement of the coverplate/electrode /liquid crystal interfacereflectivity.

For details of how to implement a wavelength-optimised coating referencemay be made to A. Deinega, I. Valuev, B. Potapkin, and Y. Lozovik,“Minimizing light reflection from dielectric textured surfaces”, J. Opt.Soc. Am. A, Vol. 28, No. 5, pp. 770-776, May 2011. In this papersimulations of the reflectivity from a variety of nano-structuredsurfaces are modelled using finite difference time domain analysis(FDTD). This technique gives a numerical solution to Maxwell's equationsfor complex surfaces by probing the structure with a plane wave in theform of a short pulse of light. As the pulse has a certain wavelengthspread, on analysis of the reflected field the reflectivity as afunction of wavelength can be derived. They model pyramidal surfacerelief structures which have a depth of d, a width of 2 L and a periodof A over a full range of wavelengths on a glass substrate (n=1.5). Weare interested in the long wavelength limit (Λ<λ) as this minimizesscattering losses as the light sees an effective graded index interface.For Λ˜λ and Λ>λ we would get scattering and diffraction losses as thelight would interact with effectively a periodic macro-prism structureon the surface of the SLM. This regime is considered in the paper, andin particular Deinega et al analyse square pyramids with linear, cubicand quantic profiles where Λ=2 L. The two key results from this paperare in FIGS. 4 and 8. FIG. 4 shows that for a given λ, as we increase d,the depth of the structure, the reflectivity reduces (ratio of d/λincreasing). This reduction depends on the pyramid profile (fivedifferent types analysed—linear pyramid, cubic profile, quantic profile,cones, and an ideal “integral” profile); our target is <0.01% or 10⁻⁴.FIG. 8 shows the reflectance map as a function of (Λ/d) and (d/λ) for asquare linear pyramidal structure.

As mentioned we are interested in the long wavelength region where λ>Λin order to minimize scattering. There are two ways to use this graph.Firstly for a varying nano-structure depth, d, and secondly for a fixednano-structure depth. Let us assume that we have two discretewavelengths, λ₁ and λ₂.

-   -   a. For a fixed value of d/Λ, if λ₂<λ₁, then (Λ/λ₁)>(Λ/λ₂). Thus        R₁>R₂. To reduce R₁, we increase d/Λ. As the graph tilts down as        d/Λ increases, R₁ decreases. Thus the pyramid depth for λ₁        should be greater than for λ₂ for the same reflectivity.    -   b. For a fixed value of d, let us consider points on the surface        map that have the same reflectivity. The aim would be to choose        the correct value for Λ₁ and Λ₂ that give the same values of R₁        and R₂ for given values of λ₁ and λ₂. For example, if Λ₁>Λ₂,        then (d/Λ₁)<(d/Λ₂), then we have two lines that run parallel to        the (d/Λ) axis. We choose them for the given wavelengths, λ₁ and        λ₂ so that R₁=R₂. This uses knowledge of the reflectivity map as        both axes depend on Λ.

As blue phase liquid crystal based SLMs do not need alignment layers,this simplifies the suppression of crosstalk from thecoverplate/ITO/liquid crystal surface as it is easier to calculate thetheoretical reflectivity of this surface and, optionally, incorporatethis in the hologram design.

Coverplate/Liquid Crystal Interface

We now consider the reflectivity due to the coverplate/liquid crystal(comprising glass, electrode (for example ITO—indium tin oxide),optional alignment layer, and liquid crystal layer(s)). This is morecomplicated because potentially four interfaces are present, and thealignment layer/liquid crystal layer reflectivity depends on the stateof the liquid crystal. However, it is possible to ‘tune out’ thecoverplate/liquid crystal reflectivity by employing a hologram patternwhich sends an equal amount of power into the +2 order 180° out-of-phasewith the power due to the coverplate/liquid crystal reflectivity. As theliquid crystal layer is very thin, temperature changes have negligibleeffect on the path length, so the reflection-cancellation hologram needsonly be determined once. This approach is possible but not so easilyimplemented when dealing the coverplate front surface reflectivity,which is ˜1 mm thick, as the same temperature change induces a largeroptical path length change. To illustrate this, the optical path lengthchange, Δopl, is given by Δopl=ndαΔT, where n is the index, d is thematerial thickness, α is the coefficient of thermal expansion, and ΔT isthe temperature change. Thus Δopl is proportional to ΔT, and maintainingthe 180° out-of-phase condition becomes progressively harder as dincreases.

FIG. 24 takes the system of FIG. 8 as an example (like elements areindicated by like reference numerals) and shows, in block diagram form,an SLM controller/driver 2400 configured to drive the LCOS SLM with sucha reflection-compensation hologram. The input data to thecontroller/driver 2400 may comprise hologram data or higher level datasuch as beam switching/routing data (which may be converted into datadefining a diffraction pattern for display on the SLM by, for example, alookup table).

The reflection-compensation, H_(R), hologram is computer generated, forexample by any of a range of standard techniques (for example aGerchberg-Saxton algorithm) and stored in non-volatile memory. This isthen added to the target hologram H_(T) to provide a combined hologramH_(C)=H_(R)+H_(T) for display.

One way to compensate for the coverplate/ITO/alignment layer/liquidcrystal interface reflectivity is, if the fundamental blazed grating hasa period of T, to add a weak blazed grating (a compensating grating) ofperiod 2T. This generates a +1 order beam that travels in the samedirection as the +2 diffracted beam of the fundamental blazed grating(which also includes the unwanted reflected light as shown in FIG. 20)with the same field value. If we move this weak blazed grating sidewaysthrough a distance of 2T the phase of the +1 order from the compensatinggrating will continuously change from 0 to 2π due to the Fourier shifttheorem. In this way it is possible adjust the phase of the compensatinggrating such that the +1 order from this grating is approximately inanti-phase with the unwanted reflected light. However, this is not agood approach in practice as the compensating grating intended tosuppress the reflection itself may introduce new noise orders that couldlead to crosstalk at other positions. A more preferable approach,particularly if one knows in advance the reflectivity of thecoverplate/ITO/liquid crystal layer and the front coverplatereflectivity is minimized, is to globally optimize a hologram (forexample along the lines described in WO2012/123713).

As previously mentioned, applying this cancellation technique to thefront coverplate surface is more difficult due to the relative thicknessof the coverplate. Temperature changes can cause the phase of thereflected beam to change with respect to the cancellation beam. Howeverthis may be compensated for by correcting the hologram for temperature,using a very thin coverplate, and/or by using a low thermal expansioncoefficient glass.

We have described, in embodiments, a phase-only LCOS device employingliquid crystal in a blue phase. It will be understood that the inventionis not limited to the described embodiments and encompassesmodifications apparent to those skilled in the art lying within thespirit and scope of the claims appended hereto.

The invention claimed is:
 1. An LCOS (liquid crystal on silicon) devicecomprising a surface bearing an anti-reflection structure, wherein: i)the anti-reflection structure comprises a physical surface having atopography with features having lateral dimensions of less than 2000 nmand having an average refraction index which decreases in a directionthat is perpendicular to said surface; and ii) a configuration of saidtopography comprising a stepped pyramid, averaged over lateraldimensions of greater than 2000 nm, varies with lateral position on saidsurface.
 2. An LCOS device as claimed in claim 1 wherein said surface isa front surface of said device.
 3. An LCOS device as claimed in claim 1,combined with a controller to display a hologram on said LCOS device todeflect first light into a first diffractive order of said hologram,wherein said hologram is further configured to deflect second light intoa second diffraction order of said hologram, wherein said LCOS devicehas an interface generating unwanted reflected light, and wherein saidsecond light is in antiphase with said unwanted reflected light.
 4. AnLCOS device as claimed in claim 3 wherein said second light hassubstantially the same power as said unwanted reflected light.
 5. TheLCOS device of claim 1 in an optical system comprising a light sourcehaving at least two different wavelengths, λ1 and λ2, wherein a firstlateral region of said surface is adapted by said topography foranti-reflection at λ1 and a second lateral region of said surface isadapted by said topography for anti-reflection at λ2.
 6. The LCOS deviceof claim 1 in an optical system comprising a wavelength-selectivedemultiplexer configured to direct light of at least two differentwavelengths, λ1, and λ2, towards different spatial regions of said LCOSdevice, wherein a first lateral region of said surface is adapted bysaid topography for anti-reflection at λ1 and a second lateral region ofsaid surface is adapted by said topography for anti-reflection at λ2. 7.A method of suppressing an unwanted reflection using the LCOS device;wherein the LCOS device comprises a surface bearing an anti-reflectionstructure, and i) the anti-reflection structure comprises a physicalsurface having a topography with features having lateral dimensions ofless than 2000 nm and having an average refraction index which decreasesin a direction that is perpendicular to said surface; and ii) aconfiguration of said topography comprising a stepped pyramid, averagedover lateral dimensions of greater than 2000 nm, varies with lateralposition on said surface, the method comprising configuring saidanti-reflection structure such that, for at least two differentwavelengths, λ1, and λ2, in a first lateral spatial region of saidsurface, reflection from said surface at λ1 is greater than at λ2, andin a second, different lateral spatial region of said surface,reflection from said surface at λ2 is greater than λ1.